1. Field of the Invention
This invention relates to a developer to be used for electrophotography, electrostatic printing, etc., particularly to a dry system developer of a capsule structure suitable for heat fixing method.
2. Description of the Prior Art
In the prior art, there have been known a large number of methods as the electrophotographic methods, as disclosed in U.S. Pat. Nos. 2,297,691, 3,666,363, and 4,071,361. Generally speaking, these methods comprise forming electrical latent images by various means on photosensitive members through utilization of photoconductive materials, then developing said latent images by use of toners, and, after optionally transferring the toner images onto transfer materials such as papers, fixing the images by heating or pressurization to obtain copied products.
Also, there have been known various developing methods in which electrical latent images are visualized by use of toners.
For example, there have been known a number of developing methods such as the magnetic brush method as disclosed in U.S. Pat. No. 2,874,063; the cascade developing method as disclosed in U.S. Pat. No. 2,618,552; the powdery cloud method and the fur brush method as disclosed in U.S. Pat. No. 2,221,776; and the liquid developing method; and so on. As the toner to be used in these developing methods, there have been conventionally used fine powders having dyes or pigments dispersed in natural or synthetic resins. Further, it is also known to use fine developing powders having incorporated the third materials for various purposes.
The developed toner-image is transferred onto a transfer material such as paper, if necessary, and fixed.
As the method for fixing toner images, there have been known the method in which toner is heated and meled by a heater or a heat roller to be fused and solidified on a support; the method in which the binder resin of toner is softened or dissolved with an organic solvent to fix the toner on a support; and the method in which the toner is fixed on a support by pressurization.
Toners are prepared from materials which are chosen so as to be suitable for respective fixing methods, and the toner to be used for a specific fixing method cannot generally be used for other fixing methods. In particular, it is almost impossible to use the toner to be used in the heat fusion fixing method by means of a heater widely practiced in the prior art for other methods such as the heat roller fixing method, the solvent fixing method or the pressure fixing method. Accordingly, research and development efforts for toners suitable for respective fixing methods has been performed.
Also, there have been known various magnetic recording methods in which magnetic latent images are formed and developed with magnetic toners.
Concerning the step of fixing toner images onto papers, etc., various methods and devices have been developed. The most general method at present is the so called hot roll fixing system in which heat and pressure are simultaneously applied, and according to this method, an image-receiving sheet carrying toner images is contacted with a heated roller to have the toner images fixed on the image-receiving sheet. However, when such a fixing system is utilized, troubles such as off-set are caused by using a toner of the prior art. Off-set is an unfavorable phenomenon in which a part of the toner carried on the image-receiving sheet is transferred to the roller surface.
As disclosed in U.S. Pat. No. 3,941,898, such an off-set phenomenon is liable to be caused in case of a developer using a low molecular weight resin. For this reason, also as disclosed in said Patent, it may be considered that such an off-set phenomenon may be prevented to some extent by use of a crosslinked resin. However, as a matter of course, by merely utilizing a crosslinked resin, the fixing temperature will be elevated to create a new problem of a low temperature off-set at the unfixed area.
The roller to be contacted with toner images is generally formed at least at its surface layer of a silicone rubber or a fluorine type resin which is good in releasability. There is also a system in which a release oil such as silicone oil is coated on the surface of the roller for prevention of off-set and for prevention of fatigue on the roller surface. However, in the system of coating an oil, there are involved such problems that provision of an oil coating system makes the fixing device complicated and that vaporization of an oil may give unpleasant feelings to users. Therefore, an approach to prevent off-set by oil coating is not preferred and under the present situation it would be very desirable to develop a toner which is broad in fixing temperature region and good in off-set resistance.
Besides the fixing characteristic, toners are also required, as a matter of course, to be excellent in other respects such as blocking resistance, developing characteristic, transfer characteristic, cleaning characteristic, and others. Toners of the prior art have one or more of the defects as mentioned below. That is, most of toners which readily melt by heating are prone to caking or agglomeration during storage or in a copying device. Most of toners are made less desirable in triboelectrification characteristic and free flow by temperature change in the environment. Also, in most toners, the mutual deterioration of toner, carrier particles and the photosensitive plate caused by collisions between toner particles and carrier particles or contact of these particles with the surface of the photosensitive plate as the result of repeated development during continuous usage, results in changed image densities or increases in the background density to lower the quality of the copied products. Thus, it would be desirable to have a toner which is excellent in various toner characteristics and suitable for the hot roller fixing method.
Further, quite recently, there has been a need for high speed fixing for the purpose of making copying operations more efficient. In the heat fixing systems of the prior art, for the purpose of enhancing the fixing speed, attempts were made to lower the softening point of the binder resin of the toner to thereby effect heat fixing more easily. However, lowering of the softening point of a resin may result in a problem such that toner particles may cause agglomeration or blocking during usage.
As described above, it has been strongly desired to have a toner which is suitable for high speed hot roller fixing, and yet excellent in toner characteristics without roller off-set and agglomeration or blocking.
In the prior art, for the purpose of merely increasing the heat fixing speed or reducing the heat source energy at a constant level of speed, it has been considered to make the toner of a capsule type, and there have been proposals about a heat fixing toner of microcapsule type aiming at a high speed heat fixing or low heat energy consumption. A capsule toner of this type has a composition of a core material of a low melting point which is more easily melted by heating and a shell material of a higher melting point which has characteristics necessary for toners such as charging characteristic, free flow, etc. For example, Japanese Patent Publication No. 1588/1974 discloses an example of a polystyrene capsule containing wax as core material or a polystyrene capsule containing an aqueous solution as core material. However, all these capsules, due to a lack of consideration about the recent developments in hot roll high speed fixing, sufer so markedly from off-set to be impractical. Thus, the heat fixing capsule toner of the prior art cannot overcome the problem of roller off-set.